The invention concerns a device for adjusting angular play on a predetermined angular range of an optical element mobile in rotation relative to a frame. It is notably applicable to driving a diffraction grid in a monochromator system.
Various mechanisms capable of driving and positioning in rotation a mobile optical element are already known, such as sinus bars. Generally, the angular range useful for rotation positioning is limited. For a diffraction grid, notably, it is usually comprised between 5xc2x0 and 60xc2x0. It is interesting, in order to control the orientation of the optical element over this angular range, to make use of a stepping motor, associated with a reduction ratio, so that the number of points on the predetermined angular range is increased. For example, whereas a stepping motor offers a few tens to approx. 400 points per revolution, the reduction ratio enables to cover 1000 points over 30xc2x0.
The driving system of the mobile optical element generates however transmission plays which are made even worse in the presence of a large reduction ratio. To obtain accurate positioning, it is therefore necessary to compensate for these plays. A known device for adjusting angular play for a monochromator system in which the diffraction grid is carried by a square, comprises a draw spring mounted on the square. This device only allows for limited rotation of the grid around its rotational axis, because the spring winds around this axis and a microswitch must be placed to avoid damaging the device in case of maloperation.
This device has the shortcoming of a mechanical transmission with limited angular travel. Moreover, the microswitch is necessary to secure the mechanism and involves additional costs. Another shortcoming of this device is that the spring applies to the square a torque that varies substantially over the angular measuring range, which is detrimental to the quality of the play adjustment,
It is an object of this invention to offer a device for adjusting angular play over a predetermined angular range that enables mechanical transmission with unlimited angular travel. This device enables therefore to do away with a microswitch.
The invention concerns such a device that is able to secure transmission at low cost and without any damage.
The device of the invention also enables to obtain a counter-torque to the driving torque with little variation over the whole predetermined angular range.
To this end, the invention applies to a device for adjusting angular play on a predetermined angular range of an optical element mobile in rotation relative to a frame.
According to the invention, the device comprises:
a contact piece integral with this element, and
an elastic steady arm with two ends.
A first of these ends is fixed to the frame. The second end cooperates with the contact piece when the mobile element is oriented in the angular range, at least when the mobile element enters this angular range in an active rotational direction, so that the steady arm exerts on the mobile element a counter-torque. This second end does not co-operate with the contact piece when the mobile element is oriented outside this angular range.
Advantageously, the rotation-mobile optical element is a diffraction grid of a monochromator, carried by a square mobile in rotation around an axis.
The aperture of the angular range is comprised between 5xc2x0 and 60xc2x0.
The orientation of the mobile element is preferably defined using a reference axis integral with the mobile element and rotating with it around the rotational axis of the element. The mobile element is then xe2x80x98orientedxe2x80x99 in the angular range when the mobile axis is in this angular range.
The device of the invention enables to exert a counter-torque selectively on the predetermined angular range and to let the mobile element rotate freely around its rotational axis outside this range, thanks to the co-operation of the contact piece and the steady arm. Thus, mechanical transmission has a limited angular travel and the play adjustment is conducted reliably on the angular range.
In certain embodiments, when the mobile element is oriented outside the angular range and enters the same, the play adjustment is performed automatically, whatever the entrance direction of the mobile element in the angular range. Thus, in a peculiar embodiment, the contact piece is arranged in the centre of the mobile element.
In other preferred embodiments, play adjustment is engaged selectively according to the entrance direction of the mobile element in the angular range. Thus, play adjustment can be controlled quite reliably and very accurately, since it is only effective when the angular range is entered in the expected direction.
Thus, preferably, the mobile element has an inactive rotational direction, the second end not co-operating with the contact piece when the mobile element enters the angular range in the inactive rotational direction.
Once the play adjustment is engaged, the mobile element having is entered the angular range in the active rotational direction, the play adjustment continues as long as the mobile element remains oriented in the angular range, regardless of its rotations (and notably their directions) within this range.
Conversely, when the mobile element enters the angular range in the inactive rotational direction, it is subject to a non-extant or very small counter-torque, regardless of its orientation in the angular range.
In the embodiments with active and inactive rotational directions, the device for adjusting play comprises advantageously a reset stop intended to block the steady arm in one displacement direction of the arm matching the inactive rotational direction of the mobile element, so that the counter-torque is greater than a threshold value as soon as the mobile element enters the angular range in the active rotational direction.
This arrangement promotes the preservation of a counter-torque with little variations over the whole angular range, whereas this torque differs from zero as soon as the mobile element enters the angular range.
In this preferred embodiment of the device for adjusting play with active and inactive rotational directions of the mobile element, the device comprises a retractable ramp integral with the mobile element and intended to co-operate with the steady arm when the mobile element enters the angular range in the inactive rotational direction, in order to prevent the steady arm from co-operating with the contact piece while conferring the counter-torque a negligible value
By xe2x80x98rampxe2x80x99 is meant a tilted element with possibly variable slope along a given direction.
The retractable ramp enables to avoid, in the inactive direction, any contact between the mobile element and the steady arm. It implies that the steady arm should be elastic not only on a plane perpendicular to the rotational axis of the mobile element, but also on a plane corresponding to the slope of the ramp. According to a peculiar embodiment in which the ramp extends between the mobile element and the steady arm, both planes are overlapping.
The retractable ramp and the steady arm are such that when the steady arm escapes from the contact piece after overshooting the end position corresponding to the active rotational direction, the steady arm returns beneath the retractable ramp. Thus, the retractable system is again ready to operate in the case of a rotation of the mobile element in the inactive direction.
Preferably, the retractable ramp is rising and extends above the contact piece.
According to other embodiments, the retractable ramp is falling or extends between the contact piece and the steady arm, while being curved towards the contact piece In another embodiment, the contact piece is rigid when it touches the steady arm in the active rotational direction and it is flexible when it touches the steady arm in the inactive rotational direction.
Advantageously, the contact piece and the steady arm are such that the counter-torque exhibits a small relative variation on the angular range, preferably smaller than 25%.
The parameters corresponding to the contact piece and to the steady arm ran be optimised for minimum variation. Indeed, the counter-torque applied depends notably on the lever arm formed by the steady arm and the contact piece, this lever arm varying according to the position of the mobile element on the angular range, as well as on the righting torque exerted on the steady arm.
Advantageously, the steady arm is coupled by its first end to a torsional spring.
The torsional spring then applies to the steady arm a righting torque that is different from the counter-torque applied by the steady arm on the mobile element. Whereas the righting torque increases substantially with the steady angle on the angular range, the contact piece and the steady arm are arranged advantageously so that the counter-torque exhibits variations with smaller relative magnitude.
As the torsional spring is formed of a winding around an axis, the steady arm consists advantageously of one end of the torsional spring, mobile in rotation around this axis.
Preferably, the torsional spring comprises contiguous spires that enable to generate automatic elastic steadying by piling up the spires.
In the embodiment with reset stop of the steady arm, this reset stop should advantageously serve simultaneously as a stop for a blocking end of the torsional spring. In another embodiment with a torsional spring, the device for adjusting play comprises two stops: one for the steady arm and another for the blocking end of the torsional spring.
In another embodiment, the steady arm is an elastic blade.
Preferably, the contact piece is a cylindrical stud. Thus, contact between the contact piece and the steady arm can be controlled efficiently.
The steady arm is preferably coupled with vertical steady means.
This embodiment is particularly interesting in the presence of a rising or falling retractable ramp. Indeed, the steady arm is then subject to vertical movements, in addition to its horizontal rotational movements, and the vertical righting means enable to bring back the steady arm to its initial position in height (advantageously horizontal).
These vertical-righting means comprise advantageously a flange containing at least one elastic element, mounted on the first end of the steady arm.
According to an advantageous embodiment of this flange, it comprises an elastic washer (for example a spring washer or a crinkle washer) co-operating with the steady arm, for example via a rigid washer.